As for the remaining 8,600 American forces, they would have left according to a gradual timeline, perhaps within 16 months.
That would have allowed Mr. Trump, who has been routinely critical of expensive American interventions in the Muslim world, to declare that he had ended a long and increasingly unpopular conflict, and to boast that he had achieved an outcome his predecessor, President Barack Obama, sought in vain.
Critics of the nascent agreement — including the former American commander in Afghanistan, the retired Army general, David H. Petraeus — had warned that it could lead to the return of Al Qaeda. Several invoked the cautionary example of Mr. Obama’s troop withdrawal from Iraq, which many national security experts blame for the 2014 emergence of the Islamic State in that country.
And in a Sept. 3 statement published by the Atlantic Council, nine former senior American diplomats with extensive experience in Afghanistan warned that a “major withdrawal of U.S. forces should follow, not come in advance of real peace agreement.” Anarchy in Afghanistan after a premature American exit “could prove catastrophic for U.S. national security” and would “underscore to potential enemies that the United States and its allies are not reliable,” the statement said.
Such critics have pointed to a recent wave of Taliban attacks as a sign that the insurgent group cannot set aside violence. The bombing cited by the president involved a car bomb detonated on Thursday at a checkpoint near the American Embassy in Kabul.
Afghan government officials who have been briefed on the negotiations privately said Mr. Khalilzad did not force enough concessions from the Taliban to ensure stability as the American military leaves Afghanistan.
One official said the agreement between Mr. Khalilzad and the Taliban would not have assured national elections on Sept. 28, as Mr. Ghani has demanded. Rather than requiring a nationwide cease-fire, it calls for a reduction of violence in Kabul and Parwan. And, the Afghan government official said, it may allow the Taliban to continue referring to itself in official conduct as the “Islamic Emirate” — as it did when the extremist group was ruling Afghanistan with fear.